1. Technical Field
The present invention relates to a high frequency power amplifier, more specifically to a high frequency power amplifier used in mobile communication devices including a mobile phone.
2. Background Art
Nowadays, mobile communication devices such as a mobile phone not only make a telephone call but also become communication means for transmitting and receiving various pieces of data such as a mail and contents. Therefore, the mobile phone is equipped with a wireless LAN (Local Area Network) or the like.
In the wireless LAN, an OFDM (Orthogonal Frequency Division Multiplexing) is used as a modulation wave in conducting high-speed data communication. Because high modulation accuracy is required in a system in which the OFDM is used, high linearity is demanded in the high frequency power amplifier. In order to meet the demand, the high frequency power amplifier is designed such that the sufficient linearity is obtained in the use output, and such that sufficiently high maximum output is obtained with respect to the use output. On the other hand, in order to increase the maximum output of the high frequency power amplifier, it is necessary to increase the number of transistors, or it is necessary to enlarge a size, which results in a delay of a thermal response of the amplifier. Therefore, immediately after the start-up of the amplifier, a time from when a temperature at the amplifier starts to increase due to heat generation of an amplifying transistor to when the temperature is stabilized is lengthened. During the increase of the temperature, a gain or a phase of the amplifier changes to fatally cause degradation of EVM (Error Vector Magnitude) in the OFDM.
For example, Unexamined Japanese Patent Publication No. 2009-200770 (hereinafter PTL 1) discloses a circuit that corrects an influence of the heat generation of the own high frequency power amplifier. That is, in starting a power of a power amplifier, a bias supplied to an amplifying transistor is transiently increased to temporarily increase the gain of the amplifier. Therefore, the time until the temperature fluctuation due to the heat generation of the amplifying transistor reaches the equilibrium state in the entire circuit is shortened in the circuit.
FIG. 6 illustrates a circuit diagram disclosed in PTL 1. A power amplifier in FIG. 6 includes amplification transistor 503, bias circuit 511, power-supply circuit 512, and speed-up circuit 522. Speed-up circuit 522 includes capacitor 521, transistor 519, and transistor 520. In the power amplifier, a charge flows into capacitor 521 during rise of a control voltage that controls on/off of power amplification. Therefore, the current is temporarily passed through collectors of transistor 519 and transistor 520, and a potential at the collector of transistor 519 is temporarily decreased. An output voltage of power-supply circuit 512 is temporarily increased to temporarily increase the bias to amplification transistor 503 from bias circuit 511. Therefore, because the gain of the amplifier is temporarily increased, the time until the temperature fluctuation due to the heat generation of the amplifying transistor reaches the equilibrium state in the entire circuit is shortened.